1. Field of the Invention
This invention relates to electronic trip units for circuit breakers, and more particularly to such trip units which respond to ground faults and sputtering arc faults, and incorporate test circuits for fully testing these features.
2. Background Information
Conventional residential and light industrial and commercial circuit breakers have a thermal trip device which responds to persistent overcurrents of moderate magnitude to provide a delayed trip, and a magnetic trip device which responds instantaneously to overcurrents of large magnitudes. Thus, the fault current must reach a pre-determined magnitude, for example, ten times rated current for the instantaneous trip to occur, or the overcurrent must sustain a predetermined average value over a given time interval to implement the delayed trip.
Recently, it has become common to incorporate ground fault trip units into such circuit breakers. The ground fault trip unit detects faults between the line conductor and ground and the neutral conductor and ground. Line-to-ground faults are commonly detected by a differential transformer in the form of a toroidal coil. The line and neutral conductors are passed through the coil so that in the absence of a line-to-ground fault, the currents are equal and opposite and no signal is generated in the secondary. A line-to-ground fault causes a sizeable imbalance in the currents in the two conductors which can be level detected. The neutral conductor is required by code to be grounded near the circuit breaker. Thus, a neutral-to-ground fault in the protected system closes a loop between the neutral conductor and ground. Typically, neutral-to-ground faults are detected by injecting a signal onto the neutral conductor which produces an oscillation if feedback is provided through the loop completed by the neutral-to-ground fault.
A common type of ground fault detector is the dormant oscillator circuit. This ground fault circuit includes two sensing coils. The first serves as a differential transformer which detects line-to-ground faults. A neutral-to-ground fault couples the two coils to provide feedback which causes an operational amplifier to oscillate.
It is common for ground fault circuit breakers to incorporate test circuits for testing the line-to-ground fault portion of the circuit. Typically, this includes a test lead which passes through the differential transformer coil and is selectively connected by a switch across the line and neutral conductors. To our knowledge, no arrangement is currently provided for testing for neutral-to-ground faults.
Recently, there has been interest in protecting electrical systems from sputtering arc faults. Sputtering arc faults occur when bare or stripped conductors come into contact. The current caused by such a fault produces magnetic repulsion forces which push the conductors apart, thereby striking an arc. The arc is extinguished as the current passes through zero. Mechanical or electro-static forces bring the conductors back into contact, and the cycle is repeated. The arc that is caused by these faults can melt the copper in the conductors, especially in stranded wire conductors such as extension cords, which can ignite surrounding materials. Although such faults present a fire hazard, the currents are typically not high enough to be detected by the thermal or magnetic trip devices.
Commonly owned U.S. Pat. No. 5,224,006 discloses a circuit breaker with an electronic trip unit which responds to sputtering arc faults. This trip unit is based on a recognition that the sputtering arc fault creates a step current wave form in contrast to the sinusoidal wave forms created by bolted faults and overload currents. Thus, the trip unit generates a bandwidth limited di/dt signal which is level detected so that sputtering arc faults with peak currents below the pick-up levels of the thermal and magnetic trip devices will trip the circuit breaker.
The sputtering arc fault detector of U.S. Pat. No. 5,224,006 utilizes the sensing coil of a dormant oscillator ground fault detector, which provides the feedback for neutral-to-ground fault detection, to generate the di/dt signal.
Commonly owned patent application Ser. No. 023,435 entitled "Circuit Breaker Responsive to Repeated In-Rush Currents Produced by a Sputtering Arc Fault" filed on Feb. 26, 1993, in the names of Raymond W. Mackenzie and Joseph C. Engel and identified as assignee's case WE 57,406 addresses the problem that the arcing wave form characterized by fast turn on to high values of current produced by sputtering arc faults is also produced by some appliances, or groups of appliances switched on simultaneously. While the magnitude of the inrush currents produced by these appliances is not as large as the currents produced by a sputtering arc fault, and thus false trips can be avoided by an appropriate setting of the level detector, this latter patent application provides discrimination between sputtering arc faults and appliance in-rush currents by recognizing that the latter are singular events while a sputtering arc fault makes and breaks repeatedly. Accordingly, the trip unit of this patent application counts the number of times that the di/dt signal exceeds the threshold within a given time interval.
The trip units described in the above patent applications both include the previously described test circuit which only tests the line-to-ground trip function. They do not test the neutral-to-ground detector or the sputtering arc detector.
There is a need there for an improved electronic trip unit which provides a complete test for ground faults, including neutral-to-ground faults, and for testing of the sputtering arc fault detector.